Semiconductor device, and display device and electronic device utilizing the same

ABSTRACT

A semiconductor device having a normal function means is provided, in which the amplitude of an output signal is prevented from being decreased even when a digital circuit using transistors having one conductivity is employed. By turning OFF a diode-connected transistor  101 , the gate terminal of a first transistor  102  is brought into a floating state. At this time, the first transistor  102  is ON and its gate-source voltage is stored in a capacitor. Then, when a potential at the source terminal of the first transistor  102  is increased, a potential at the gate terminal of the first transistor  102  is increased as well by bootstrap effect. As a result, the amplitude of an output signal is prevented from being decreased.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/941,739, filed Nov. 16, 2015, now allowed; which is a continuation ofU.S. application Ser. No. 14/472,748, filed Aug. 29, 2014, now U.S. Pat.No. 9,190,425; which is a continuation of U.S. application Ser. No.13/906,934, filed May 31, 2013, now U.S. Pat. No. 8,823,620, which is acontinuation of U.S. application Ser. No. 13/277,301, filed Oct. 20,2011, now U.S. Pat. No. 8,456,402; which is a continuation of U.S.application Ser. No. 12/849,885, filed Aug. 4, 2010, now U.S. Pat. No.8,044,906; which is a continuation of U.S. application Ser. No.11/675,122, filed Feb. 15, 2007, now U.S. Pat. No. 7,786,985; which is acontinuation of U.S. application Ser. No. 10/740,840, filed Dec. 22,2003, now U.S. Pat. No. 7,202,863; which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2002-374098 on Dec.25, 2002, all of which are incorporated by reference.

INDUSTRIAL FIELD FOR THE INVENTION

The present invention relates to a configuration of a digital circuit.More particularly, the invention relates to a technology for amplifyingan output signal even larger by using a bootstrap circuit, and furtherto a display device, a semiconductor device or an electronic device eachusing the technology.

BACKGROUND ART

In recent years, display devices in which a semiconductor thin film isformed on an insulator such as a glass substrate, in particular activematrix display devices using thin film transistors (hereinafter referredto as TFTs), have been in widespread use in various fields. An activematrix display device using TFTs has several hundred thousand to severalmillion pixels arranged in matrix, and it displays images by controllingthe electric charge in each pixel by using a TFT disposed in each pixel.

As a recent technology, the technology relating to poly-silicon TFTs inwhich a driver circuit is simultaneously formed in a peripheral regionof a pixel portion in addition to TFTs which constitute pixels, has beendeveloped. This technology contributes greatly to reducing devices insize and electric power consumption. Display devices have thus becomeindispensable devices to be used for display portions of mobileinformation terminals and the like, application fields of which areexpanding at remarkable speed.

As the driver circuit of the display device, a CMOS circuit in which anN-channel TFT and a P-channel TFT are combined is usually adopted. TheCMOS circuit has advantages in that it can suppress the consumed currentin the whole circuit and perform high speed driving since a currentflows only at an instant when logic is changed and a current does notflow during a period in which a certain logic is held (as there is onlya minute leak current in practice).

As mobile electronic devices are reduced in size and weight, demand fora display device using a self-light emitting element and a liquidcrystal element and the like such as an organic EL element, an FED(Field Emission Display), and an element used for a liquid crystaldisplay is rapidly increasing; however, from the viewpoint of the yieldand the like, it is difficult to reduce the manufacturing cost to thelevel sufficiently low since the great many number of TFTs are required.It is easily supposed that the demand is further rapidly increased infuture, and therefore, it is desired that the display device can besupplied more inexpensively.

As a method of fabricating a driver circuit on an insulator, there is acommon method in which patterns of active layers, wirings and the likeare formed through exposure treatment and etching with a plurality ofphotomasks. Since the number of manufacturing steps is a dominant factorin determining the manufacturing cost, a manufacturing method using assmall number of manufacturing steps as possible is ideal formanufacturing driver circuits. Thereupon, a driver circuit, which isconventionally configured by the CMOS circuit, is configured by usingTFTs which have either N-channel type or P-channel type conductivity.With this method, a part of an ion doping step can be omitted, and thenumber of the photomasks can also be reduced. Therefore, the costreduction is achieved.

FIG. 9A shows an example of a TFT load-type inverter circuit formed byusing TFTs having only one conductivity. The operation thereof isdescribed below.

FIG. 9B shows the waveform of a signal input to the inverter circuit.The input signal amplitude is between a high potential side power supplyVDD and a low potential side power supply GND. It is assumed that GND=0V for simplicity.

The circuit operation is described now. To describe the operation simplyand explicitly, the threshold voltages of N-channel type TFT whichconfigure the circuit have no variations and represented by (VthN)across the board, and the threshold voltages of P-channel type TFTs aresimilarly represented by a constant value (VthP).

When a signal as shown in FIG. 9B is input to the inverter circuit andthe input signal is an L signal (low potential side power supply GND),an N-channel type TFT 904 is turned OFF. Meanwhile, a potential at anoutput terminal is pulled up toward a high potential side power supplyVDD since a load TFT 903 operates in a saturated region at all times. Onthe other hand, when the input signal is an H signal (high potentialside power supply VDD), the N-channel type TFT 904 is turned ON. Thepotential at the output node is pulled down toward the low potentialside power supply GND if the current capacity of the n-channel type TFT904 is set sufficiently larger than that of the load TFT 903.

However, there is the following problem in this case. FIG. 9C shows thewaveform of the output from the TFT load-type inverter circuit. When theinput signal is at L level, the potential at the output terminal islower than VDD by an amount denoted by 907, namely by a thresholdvoltage of the load TFT 903 as shown in FIG. 9C. This is because fewcurrent flows in the load TFT 903 when the gate-source voltage of theload TFT 903 is smaller than the threshold voltage, thus the load TFT903 is turned OFF. The source terminal of the load TFT 903 is an outputterminal and the gate terminal thereof is connected to VDD here.Therefore, the potential at the output terminal is lower than thepotential at the gate terminal by the threshold voltage. That is, thepotential at the output terminal can be increased to be (VDD−VthN) athighest. Further, when the input signal is an H signal, the potential atthe output terminal is higher than GND by an amount denoted by 908,depending on the ratio of the current capacities of the load TFT 903 tothe n-channel type TFT 904. To bring the output potential sufficientlyclose to GND, it is necessary to sufficiently increase the currentcapacity of the n-channel type TFT 904 relatively to that of the loadTFT 903.

That is, when using the above-described inverter circuit formed by usingTFTs having only one conductivity, the amplitude of the output signal isattenuated relative to the amplitude of the input signal.

Hereupon, several methods for avoiding the problem that the amplitude ofan output signal is attenuated have been studied (see Patent Documents 1to 4 for example).

FIG. 33 shows a circuit diagram of an inverter circuit shown in PatentDocuments 1 and 2. The circuit shown in FIG. 33 has the advantage thatwhen the gate terminal of a transistor 3302 is brought into a floatingstate, a voltage at both terminals (potential difference between bothterminals) of a capacitor 3304 does not change.

The operation of FIG. 33 is described next. A pair of signals invertedfrom each other is input to each of input terminals 3305 and 3306.First, an H signal (high potential side power supply VDD) is input tothe input terminal 3306 and an L signal (low potential side power supplyGND) is input to the input terminal 3305. Then, a transistor 3303 isturned ON and a potential at a terminal 3308 becomes equal to thepotential of the L signal (low potential side power supply GND).Meanwhile, a transistor 3301 is turned ON as the potential at the inputterminal 3305 is equal to the potential of the L signal (low potentialside power supply GND). As a result, a terminal 3307 becomes equal tothe potential of the L signal (low potential side power supply GND).That is, the voltage at both terminals (potential difference betweenboth terminals) of the capacitor 3304 becomes equal to 0 V.

Next, when an H signal (high potential side power supply VDD) is inputto the input terminal 3305 and an L signal (low potential side powersupply GND) is input to the input terminal 3306, the transistor 3303 isturned OFF. Since the potential at the input terminal 3305 is equal tothe potential of the H signal (high potential side power supply VDD),the transistor 3301 is turned ON and thus the potential at the terminal3307 is increased. When the gate-source voltage of the transistor 3302becomes higher than the threshold voltage, the transistor 3302 is turnedON, and a potential at the terminal 3308 starts increasing. In such acase, when the potential at the terminal 3307 keeps on increasing, thetransistor 3301 is turned OFF at the end. This is because, as theterminal 3307 corresponds to the source terminal of the transistor 3301,the gate-source voltage of the transistor 3301 becomes smaller when thepotential at the terminal 3307 is increased, thus reaches the thresholdvoltage at the end. When the gate-source voltage of the transistor 3301becomes equal to the threshold voltage, the transistor 3301 is turnedOFF. Therefore, the current flow from the terminal 3305 to the terminal3307 is cut off. That is, the terminal 3307 is brought into a floatingstate. As a result, the voltage at both terminals (potential differencebetween both terminals) of the capacitor 3304 does not change any more.

In the case where the potential at the terminal 3308 still keeps onincreasing at the point when the transistor 3301 is turned OFF, thetransistor 3302 is ON. That is, the gate-source voltage of thetransistor 3302, namely the voltage at both terminals (potentialdifference between both terminals) of the capacitor 3304 is larger thanthe threshold voltage of the transistor 3302. Therefore, the potentialat the terminal 3308 is further increased. At this time, the potentialat the terminal 3307 is also increased. This is because, when thepotential at either terminal of the capacitor 3304 (the terminal 3308)is increased, the potential at the other terminal (the terminal 3307) isalso increased since the voltage at both terminals (potential differencebetween both terminals) of the capacitor 3304 does not change any more.Thus, the potential at the terminal 3308 keeps on increasing and reachesthe high potential side power supply VDD at the end. While the potentialat the terminal 3308 is increasing until it reaches the high potentialside power supply VDD, the transistor 3302 is constantly ON. Thecapacitor 3304 holds the very voltage at which the transistor 3301 isturned OFF. Therefore, the potential at the terminal 3307 is higher thanthe high potential side power supply VDD by the voltage which is storedin the capacitor 3304.

That is, the potentials at the terminals 3307 and 3308 are equal to orhigher than the high potential side power supply VDD. Thus, it can beprevented that the amplitude of the output signal becomes smaller thanthat of the input signal.

Such a circuit is generally referred to as a bootstrap circuit.

[Patent Document 1] Japanese Patent Laid-Open No. Hei 8-50790

[Patent Document 2] Japanese Patent No. 3330746 Specification

[Patent Document 3] Japanese Patent No. 3092506 Specification

[Patent Document 4] Japanese Patent Laid-Open No. 2002-328643

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, there are two major problems in the inverter circuit shown inFIG. 33.

The first problem is that when an H signal (high potential side powersupply VDD) is input to the input terminal 3305 and an L signal (lowpotential side power supply GND) is input to the input terminal 3306,the potentials at the terminals 3307 and 3308 are not increasedsufficiently in the case where the transistor 3301 is turned OFF late.If the transistor 3302 is turned OFF first, the capacitor 3304accumulates the threshold voltage of the transistor 3302 as it isdisposed between the gate and the source of the transistor 3302. At thistime, the potential at the terminal 3307 is still on the increase sincethe transistor 3301 is ON. When the transistor 3301 is turned OFF, thethreshold voltage of the transistor 3302 is held in the capacitor, andthe transistor 3302 is turned OFF. Thus, the potentials at the terminals3308 and 3307 do not increase any more.

The second problem is that when the potential of an H signal input tothe input terminal 3305 is lower than the high potential side powersupply VDD, the potentials at the terminals 3307 and 3308 are notincreased sufficiently. In the case of employing the circuit as shown inFIG. 9A as the circuit for outputting a signal to the input terminal3305, the potential of the H signal may be lower than the high potentialside power supply VDD. Hereupon, suppose the case where the differencebetween the potential of the H signal and the high potential side powersupply VDD is higher than the threshold voltage of the transistor 3301.In such a case, the transistor 3301 is not turned OFF even if thepotential increase at the terminal 3307 terminates when the H signal isinput to the input terminal 3305 and the L signal (low potential sidepower supply GND) is input to the input terminal 3306. That is, theterminal 3307 is not brought into a floating state, and the electriccharge is kept on being supplied from the terminal 3305 to the terminal3307. Therefore, the potentials at the terminals 3305 and 3307 aremaintained to be equal to each other. Thus, operation such as the one inwhich the voltage at both terminals (potential difference between bothterminals) of the capacitor 3304 does not change is not brought on. As aresult, the potentials at the terminals 3307 and 3308 are not increasedsufficiently.

When connecting the output terminal of the inverter circuit as describedabove to another inverter circuit of the similar configuration, thesignal amplitude of the output terminal becomes even lower. That is, asthe larger number of circuits is connected to the output terminal of theinverter circuit, the amplitude of the output signal becomes smaller.Thus the normal circuit operation is not achieved.

On the other hand, according to the inverter circuit shown in PatentDocument 4, the aforementioned second problem is solved. FIG. 34 showsan inverter circuit shown in Patent Document 4. When an H signal whichis lower than the high potential side power supply VDD is input to aninput terminal 3405, and an L signal (low potential side power supplyGND) is input to an input terminal 3406, a potential at a terminal 3407is increased. When the gate-source voltage of a transistor 3401 becomesequal to the threshold voltage, the transistor 3401 is turned OFF. Thatis, the terminal 3407 is brought into a floating state. Thus, a voltageat both terminals (potential difference between both terminals) of acapacitor 3404 at this time is stored. Therefore, if a transistor 3402is ON at the point when the transistor 3401 is turned OFF, a potentialat a terminal 3408 keeps on increasing, and the potential at theterminal 3407 is also increased as a result.

However, the first problem mentioned above is not solved even when usingthe circuit shown in FIG. 34.

In view of the foregoing problem, it is an object of the invention toprovide a semiconductor device in which the amplitude of an outputsignal does not easily become smaller. It is another object of theinvention to provide a semiconductor device in which a circuit can beconfigured by using transistors having only one conductivity.

It is to be noted that a semiconductor device means a device whichincludes a circuit having a semiconductor element (transistor anddiode), a capacitor, a resistor and the like. It is needless to mentionthat the invention is not limited to these elements.

Means for Solving the Problem

The present invention uses the following means to solve theaforementioned problems.

A semiconductor device according to the invention includes first tothird transistors and first and second input terminals, wherein thesource terminal of the first transistor is connected to the drainterminal of the second transistor, the drain terminal of the thirdtransistor is connected to the gate terminal of the first transistor,the first input terminal is connected to the gate terminal of the thirdtransistor and the gate terminal of the second transistor, and thesecond input terminal is connected to the gate terminal of the firsttransistor through a rectifying element.

In addition, according to the above configuration of the semiconductordevice of the invention, the rectifying element is a diode-connectedtransistor.

That is, according to the invention, the rectifying element such as adiode-connected transistor is connected to a signal input portion.

By turning OFF the diode-connected transistor, the gate terminal of thefirst transistor is brought into a floating state. At this time, thefirst transistor is ON, and its gate-source voltage is stored in acapacitor (gate capacitance of the transistor). Subsequently, when apotential at the source terminal of the first transistor is increased, apotential at the gate terminal of the first transistor is increased aswell due to bootstrap effect. As a result, the amplitude of an outputsignal is prevented from being decreased.

In addition, according to the above configuration of the semiconductordevice of the invention, the third transistor is connected in series toa second rectifying element.

In addition, according to the above configuration of the semiconductordevice of the invention, the second rectifying element is adiode-connected transistor.

That is, the second rectifying element such as a diode-connectedtransistor is connected to the gate terminal portion of the firsttransistor.

By turning OFF the diode-connected transistor as the second rectifyingelement, a potential at the gate terminal of the first transistor isprevented from dropping to a large degree. As a result, the amplitude ofan output signal is prevented from being decreased.

In addition, according to the above configuration of the semiconductordevice of the invention, the diode-connected transistor and the firsttransistor have the same conductivity.

That is, by adopting transistors having the same conductivity for boththe first transistor and the diode-connected transistor, all thetransistors configuring the circuit can have the same conductivity. As aresult, cost reduction can be achieved.

In addition, according to the above configuration of the semiconductordevice of the invention, the diode-connected transistor as the secondrectifying element and the first transistor have the same conductivity.

That is, by adopting transistors having the same conductivity for boththe first transistor and the diode-connected transistor as the secondrectifying element, the threshold voltage of each transistor can be setroughly the same. Since the threshold voltage of the first transistor isalmost equal to that of the diode-connected transistor as the secondrectifying element, it is prevented that current leaks when the firsttransistor is required to be turned OFF.

In addition, according to the above configuration of the semiconductordevice of the invention, a capacitor is provided, one of which isconnected to the gate terminal of the first transistor and the otherterminal thereof is connected to the source terminal of the firsttransistor.

It is to be noted that the transistor of the invention may be formed byany types of material, means and manufacturing method, and any types oftransistor can be employed. For example, it may be, a thin filmtransistor (TFT). Among TFTs, a TFT having an amorphous, polycrystalline or single crystalline semiconductor layer may be adopted. Asan alternative transistor, a transistor formed on a single crystallinesubstrate, an SOI substrate, a plastic substrate or a glass substratemay be adopted. Further, a transistor formed of an organic material or acarbon nanotube may be adopted as well. A MOS type transistor or abipolar transistor may also be employed.

It is to be noted that according to the invention, connection means anelectrical connection. Therefore, other elements or circuits and thelike may be interposed between the shown elements.

According to the configuration of the invention, either terminal of acapacitor configuring a bootstrap circuit is easily brought into afloating state. As a result, the amplitude of an output signal isprevented from being decreased. In addition, even when the amplitude ofan input signal is small, either terminal of the capacitor configuringthe bootstrap circuit can be brought into a floating state. Therefore,the amplitude of an output signal is prevented from being decreased.Further, as the circuit can be configured by using transistors of onlyone conductivity, the manufacturing cost can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the configuration of a circuit in the caseof applying the invention to an inverter circuit;

FIG. 2 is a diagram showing the configuration of a circuit in the caseof applying the invention to an inverter circuit;

FIG. 3 is a diagram showing a symbol expressing an inverter circuit towhich the invention is applied;

FIG. 4 is a diagram showing the configuration of a circuit in the caseof applying the invention to an inverter circuit;

FIG. 5 is a diagram showing the configuration of a circuit in the caseof applying the invention to an inverter circuit;

FIG. 6 is a diagram showing the configuration of a circuit in the caseof applying the invention to an inverter circuit;

FIG. 7 is a diagram showing the configuration of a circuit in the caseof applying the invention to an inverter circuit;

FIG. 8 is a diagram showing the configuration of a circuit in the caseof applying the invention to an inverter circuit;

FIG. 9A to 9C are diagrams showing the configuration and operations of aconventional inverter circuit;

FIG. 10 is a diagram showing the configuration of a circuit in the caseof applying the invention to a clocked inverter circuit;

FIG. 11 is a diagram showing a symbol expressing a clocked invertercircuit;

FIG. 12 is a diagram showing the configuration of a circuit in the caseof applying the invention to a NAND circuit;

FIG. 13 is a diagram showing a symbol expressing a NAND circuit to whichthe invention is applied;

FIG. 14 is a diagram showing the configuration of a circuit in the caseof applying the invention to a NOR circuit;

FIG. 15 is a diagram showing the configuration of a circuit in the caseof applying the invention to a transfer gate circuit;

FIG. 16 is a diagram showing the configuration of a circuit in the caseof applying the invention to an inverter circuit;

FIG. 17 is a diagram showing the configuration of a circuit in the caseof applying the invention to a clocked inverter circuit;

FIG. 18 is a diagram showing the configuration of a circuit in the caseof applying the invention to a NAND circuit;

FIG. 19 is a diagram showing the configuration of a circuit in the caseof applying the invention to a NOR circuit;

FIG. 20 is a diagram showing the configuration of a circuit in the caseof applying the invention to a transfer gate circuit;

FIG. 21 a diagram showing the configuration of a circuit in the case ofapplying the invention to an inverter circuit;

FIG. 22 is a diagram showing a symbol expressing an inverter circuit towhich the invention is applied;

FIG. 23 is a diagram showing the configuration of a circuit in the caseof applying the invention to a clocked inverter circuit;

FIG. 24 is a diagram showing a symbol expressing a clocked invertercircuit to which the invention is applied;

FIG. 25 is a diagram showing the configuration of a circuit in the caseof applying the invention to a NAND circuit;

FIG. 26 is a diagram showing a symbol expressing a NAND circuit to whichthe invention is applied;

FIG. 27 is a diagram showing the configuration of a circuit in the caseof applying the invention to an inverter circuit;

FIG. 28 is a diagram showing the configuration of a display device ofthe invention;

FIG. 29 is a diagram showing the configuration of a circuit in the caseof applying the invention to a DFF circuit;

FIG. 30 is a diagram showing the configuration of a circuit in the caseof applying the invention to a DFF circuit;

FIG. 31 is a diagram showing the configuration of a circuit in the caseof applying the invention to a shift register;

FIGS. 32 A to 32H are diagrams showing electronic devices to which theinvention is applied;

FIG. 33 is a diagram showing the configuration of a conventionalinverter circuit;

FIG. 34 is a diagram showing the configuration of a conventionalinverter circuit.

EMBODIMENT MODES OF THE INVENTION

The circuit configuration of a semiconductor device of the inventionwill be hereinafter described.

Embodiment Mode 1

First, described in this embodiment mode is an inverter circuit fordealing with the second problem as described in the section of theproblems to be solved by the invention. That is, described here is theinverter circuit for dealing with the problem that a potential at acertain terminal are not increased sufficiently in the case where apotential of an H signal which is input to an input terminal is lowerthan a high potential side power supply VDD.

FIG. 2 shows an inverter circuit in which potentials at terminals 107and 108 can be increased sufficiently even when a potential of an Hsignal which is input to an input terminal 105 is lower than a highpotential side power supply VDD. The input terminal 105 is connected tothe gate terminal of a transistor 102 through a diode-connectedtransistor 101. Since the transistor 101 is diode connected, its gateterminal is connected to the input terminal 105. Therefore, current canflow in the direction from the terminal 105 to the terminal 107 whilenot in the direction from the terminal 107 to the terminal 105. Inaddition, a capacitor 104 is connected between the gate terminal and thesource terminal of the transistor 102. The drain terminal of atransistor 103 is connected to the source terminal of the transistor102, and the gate terminal of the transistor 103 is connected to aninput terminal 106. The gate terminal of a transistor 109 is connectedto the input terminal 106, and the drain terminal thereof is connectedto the gate terminal of the transistor 102.

It is to be noted that although the source terminal of the transistor109 and the source terminal of the transistor 103 are connected to a lowpotential side power supply GND, the invention is not limited to this.Each source terminal may be connected to a wiring of a differentpotential, or a pulse signal may be input to the terminal.

Also, although the input terminal 106 is connected to the gate terminalof the transistor 109 and the gate terminal of the transistor 103, theinvention is not limited to this. Each gate terminal may be connected toa different input terminal.

In addition, although the drain terminal of the transistor 102 isconnected to the high potential side power supply VDD, the invention isnot limited to this. It may be connected to a wiring of a differentpotential or a pulse signal may be input to it.

The operation of FIG. 2 is described now. A pair of signals invertedfrom each other is input to each of the input terminals 105 and 106.However, it is also possible to operate the circuit without inputting aninverted signal at all times. First, an H signal (high potential sidepower supply VDD) is input to the input terminal 106, and an L signal(low potential side power supply GND) is input to the input terminal105. Then, the transistors 109 and 103 are turned ON. As a result, apotential at the terminal 108 becomes equal to GND. Since a potential atthe terminal 107 becomes equal to GND, the transistor 102 is turned OFF.In addition, since the terminals 105 and 107 have the same potential,the transistor 101 is turned OFF. Also, a voltage at both terminals(potential difference between both terminals) of the capacitor 104becomes equal to 0 V.

Next, an H signal (high potential side power supply VDD) is input to theinput terminal 105 and an L signal (low potential side power supply GND)is input to the input terminal 106. Then, the transistors 109 and 103are turned OFF. Since a potential at the input terminal 105 is equal tothe potential of the H signal (high potential side power supply VDD),the transistor 101 is turned ON and the potential at the terminal 107 isincreased. When the gate-source voltage of the transistor 102 becomeslarger than the threshold voltage, the transistor 102 is turned ON and,the potential at the terminal 108 starts increasing. In such a case, asthe potential at the terminal 107 keeps on increasing, the transistor101 is turned OFF at the end. This is because, since the terminal 107corresponds to the source terminal of the transistor 101, thegate-source voltage (drain-source voltage) of the transistor 101 dropswhen the potential at the terminal 107 is increased, thus it reaches thethreshold voltage at the end. When the gate-source voltage of thetransistor 101 becomes equal to the threshold voltage, the transistor101 is turned OFF. Therefore, the current flow from the terminal 105 tothe terminal 107 is cut off. That is, the terminal 107 is brought into afloating state. As a result, the voltage at both terminals (potentialdifference between both terminals) of the capacitor 104 does not changeanymore.

If the potential at the terminal 108 is still on the increase at thepoint when the transistor 101 is turned OFF, the transistor 102 is ON.That is, the gate-source voltage of the transistor 102, namely thevoltage at both terminals (potential difference between both terminals)of the capacitor 104 is larger than the threshold voltage of thetransistor 102. Thus, the potential at the terminal 108 further keeps onincreasing. At this time, the potential at the terminal 107 is increasedas well. This is because, since the voltage at both terminals (potentialdifference between both terminals) of the capacitor 104 does not changeanymore, when a potential at either terminal (the terminal 108) of thecapacitor 104 is increased, the other terminal thereof (the terminal107) is also increased. The potential at the terminal 108 further keepson increasing, and it reaches the high potential side power supply VDDat the end. Until the potential at the terminal 108 reaches the highpotential side power supply VDD, the transistor 102 is constantly ON.The capacitor 104 stores the very voltage at the point when thetransistor 101 is turned OFF. Therefore, the potential at the terminal107 is higher than the high potential side power supply VDD by thevoltage which is stored in the capacitor 3304.

That is, each of the potentials at the terminals 107 and 108 is equal toor more than the high potential side power supply VDD. Therefore, aproblem such that the amplitude of an output signal becomes smaller thanthat of an input signal can be prevented.

As described above, the signal which is input to the terminal 106 isinverted in the terminals 107 and 108. Thus, in the inverter circuitshown in FIG. 2, the input terminal corresponds to the terminal 106, andthe output terminal corresponds to the terminal 107 or 108. The terminal105 may be input with an inverted signal of the signal at the terminal106. Therefore, the terminal 105 may be included in the input terminals.

Whether to output a signal to the terminal 107 or to the terminal 108may be determined by the size of input impedance of a circuit which isconnected next to the inverter circuit. That is, the terminal 107 isrequired to be in a floating state depending on the operating condition.Therefore, the terminal 107 cannot be connected to a circuit having lowinput impedance. However, the potential at the terminal 107 can be sethigher than VDD when an H signal is input to it. Meanwhile, the terminal108 can be connected to a circuit of which input impedance is not lowsince the terminal 108 does not need to be in a floating state. However,when an H signal is input to the terminal 108, the potential at theterminal 108 does not become higher than VDD. As described above, sinceeach terminal has differences, whether to output a signal to theterminal 107 or the terminal 108 may be determined appropriately.

FIG. 3 shows a symbol 301 representing the inverter circuit shown inFIG. 2. An input terminal 303 corresponds to the terminal 106 and aninput terminal 304 corresponds to the terminal 105. An output terminal302 corresponds to the terminal 108 or the terminal 107. A pair ofsignals inverted from each other is input to each of the terminal 303and the terminal 304. Taking account of its operation as an invertercircuit, a signal which is input to the terminal 303 is inverted andoutput to the output terminal 302. Thus, the terminal 303 corresponds tothe input terminal in an inverter circuit.

Described herein is the case where the potential of an H signal which isinput to the input terminal 105 is lower than the high potential sidepower supply VDD. Suppose the case in which the difference between thepotential of an H signal which is input to the input terminal 105 andthe high potential side power supply VDD is higher than the thresholdvoltage of the transistor 101. Even in such a case, when an H signal isinput to the input terminal 105 and an L signal (a low potential sidepower supply GND) is input to the input terminal 106, the potential atthe terminal 107 is increased and the gate-source voltage of thetransistor 101 reaches the threshold voltage, and then, the transistor101 is turned OFF, thus the terminal 107 is brought into a floatingstate. Therefore, if the transistor 102 is ON at the point when thetransistor 101 is turned OFF, the gate-source voltage of the transistor102 is held in the capacitor 104. Therefore, the potentials at theterminals 108 and 107 are increased sufficiently.

As described above, even a general CMOS circuit in which theconductivity of P-channel type transistors is inverted can operatenormally by using the transistors 101 and 109, the capacitor 104 and thelike. This can be applied to various circuits as well as an invertercircuit.

It is to be noted that although the drain terminal of the transistor 102in FIG. 2 is connected to a wiring having the potential VDD, theinvention is not limited to this. The potential at the drain terminal ofthe transistor 102 may be changed depending on the condition. Forexample, it may be input with a pulse signal. Similarly, although eachof the source terminals of the transistors 103 and 109 is connected to awiring having the potential GND, the invention is not limited to this.Each of the potentials at the source terminals of the transistors 103and 109 may be changed depending on the condition, and it may be inputwith a different potential or a signal.

For example, the drain terminal of the transistor 102 may be connectedto the input terminal 105 of the transistor 102 as shown in FIG. 4. Inthis case also, when an H signal (high potential side power supply VDD)is input to the input terminal 106 and an L signal (low potential sidepower supply GND) is input to the input terminal 105, the potential atthe output terminal 108 becomes equal to GND, and when an L signal (lowpotential side power supply GND) is input to the input terminal 106 andan H signal (high potential side power supply VDD) is input to the inputterminal 105, the potential at the output terminal 108 becomes equal toVDD. Thus, the circuit operates normally.

Alternatively, when a pulse signal is input to the drain terminal of thetransistor 102, a shift register, a latch circuit, or a part of them canbe configured.

It is to be noted that although the N-channel transistors are employedin FIG. 2, the invention is not limited to this. P-channel transistorsmay be employed to configure the circuit, or a CMOS circuit may beemployed as well. When adopting P-channel transistors for all of thetransistors in the circuit shown in FIG. 2, the potentials of VDD andGND may be replaced with each other.

Although the transistor 101 shown in FIG. 2 has the same conductivity asthe transistor 102 and the like, the invention is not limited to this.Any element having rectification may be adopted. For example, instead ofthe transistor 101, a PN junction diode, a PIN junction diode, or aSchottky diode and the like may be adopted. Alternatively, as shown inFIG. 5, a diode-connected transistor 101P whose conductivity is oppositeto the transistor 102 and the like may be adopted as well.

In addition, the capacitor 104 may be omitted. That is, it can besubstituted with the gate capacitance of the transistor 102. As for thegate capacitance of the transistor 102, it may be formed in anoverlapped region of the gate electrode with a source region, a drainregion, an LDD region and the like, or formed between the gate electrodeand a channel region.

Embodiment Mode 2

In Embodiment Mode 1, the inverter circuit for dealing with the secondproblem described in the section of the problems to be solved by theinvention is described. Described in this embodiment mode is an invertercircuit for dealing with the first problem described therein.

Now, a factor of the first problem is analyzed with reference to thecircuit in FIG. 33 again. When an H signal (high potential side powersupply VDD) is input to the input terminal 3306 and an L signal (lowpotential side power supply GND) is input to the input terminal 3305,the potential at the terminal 3307 becomes equal to the potential of theL signal (low potential side power supply GND). That is, the voltage atboth ends (potential difference between both ends) of the capacitor 3304becomes equal to 0 V.

Next, when an H signal (high potential side power supply VDD) is inputto the input terminal 3305 and an L signal (low potential side powersupply GND) is input to the input terminal 3306, the potential at theterminal 3307 starts increasing from GND (0 V). Then, when the potentialat the terminal 3307 becomes equal to (VDD−VthN) which is lower than VDDby the threshold voltage, it is brought into a floating state. That is,such amount of the potential difference is required to be increased.Therefore, the corresponding charge time has to be provided. This causesthe delay of the terminal 3307 to be in the floating state.

Hereupon, according to the invention, the circuit operates withoutdecreasing the potential at the terminal 3307 (or a terminalcorresponding to this) down to GND (0 V). However, when a transistor isrequired to be turned OFF, the potential at the terminal drops to thevicinity of the threshold voltage. As a result, not 0 V but thethreshold voltage is stored in the capacitor. Thus, a potential does nothave to be increased by a large amount since electric charge is alreadyheld in the capacitor. Therefore, the charge time is reduced, thus thetime required for bringing the terminal into the floating state isreduced as well.

Based on the principle as described above, the circuit is configured todeal with the first problem.

In this embodiment mode, the first problem is solved by modifying thecircuit described in Embodiment Mode 1. Therefore, the first and secondproblems can be solved at the same time. Thus, the detailed descriptionof the basic configuration and operation is omitted herein as it is thesame as in Embodiment Mode 1.

FIG. 1 shows a circuit diagram in which the circuit in FIG. 2 ismodified to solve both of the first and second problems. In FIG. 1, atransistor 110 which is diode connected (whose gate terminal and drainterminal are connected to each other) is connected in series to thetransistor 109 to solve the second problem described in the section ofthe problems to be solved by the invention. It is to be noted thatalthough the transistor 110 is connected to the drain terminal side ofthe transistor 109, the invention is not limited to this. For example,the transistor 110 may be connected to the source terminal side of thetransistor 109 as shown in FIG. 6.

By disposing the diode-connected transistor 110 as shown in FIG. 1, itbecomes possible to prevent the potential at the terminal 107 from beinglower than the threshold voltage. That is, the voltage at both terminals(potential difference between both ends) of the capacitor 104 can be setto be not equal to 0 V, but equal to or more than the threshold voltage.

The operation thereof is described now in brief. First, when an H signal(high potential side power supply VDD) is input to the input terminal106 and an L signal (low potential side power supply GND) is input tothe input terminal 105, the transistors 109 and 103 are turned ON. As aresult, the potential at the terminal 108 becomes equal to GND. On theother hand, the potential at the terminal 107 becomes equal to thethreshold voltage of the transistor 110 since the transistor 101 is OFF,and the transistor 110 is turned OFF when the source-drain voltage ofthe transistor 110 becomes equal to the threshold voltage as the gateterminal and the drain terminal of the transistor 110 are connected toeach other. The voltage at both terminals (potential difference betweenboth terminals) of the capacitor 104 also becomes equal to the thresholdvoltage as the potential at the terminal 107 is equal to the thresholdvoltage. Therefore, when the threshold voltage of the transistor 110 isequal to that of the transistor 102, the transistor 102 is turned OFF.

Next, when an H signal (high potential side power supply VDD) is inputto the input terminal 105 and an L signal (low potential side powersupply GND) is input to the input terminal 106, the transistors 109 and103 are turned OFF. Since the potential at the input terminal 105 hasthe potential of the H signal (high potential side power supply VDD),the transistor 101 is turned ON and the potential at the terminal 107 isthus increased. The potential at the terminal 107 starts increasing fromthe threshold voltage in FIG. 1, whereas it starts increasing from GND(0 V) in FIG. 2. Thus, the potential at the terminal 107 is increasedinstantly. As a result, the transistor 101 is instantly turned OFF, andthus the terminal 107 is brought into the floating state. At this point,the transistor 102 is ON since the potential at the terminal 108 isstill on the increase. Therefore, the problem that the potentials at theterminals 108 and 107 are not increased sufficiently can be solved.

Due to the transistor 110, the potential change at the terminal 107 canbe suppressed small, thus the potential can change instantly. Thiscontributes to the faster circuit operation.

By adopting such a configuration, the first and second problemsdescribed in the section of the problems to be solved by the inventioncan be solved at the same time.

It is to be noted although N-channel type transistors are employed inFIGS. 1 and 6, the invention is not limited to this. When employingP-channel type transistors for all of the transistors in the circuits inFIGS. 1 and 6, the potentials of VDD and GND may be replaced with eachother. FIG. 7 shows a circuit diagram in the case where all thetransistors in the circuit in FIG. 1 are P-channel transistors.

It is also to be noted that although the transistor 110 in FIGS. 1 and 6has the same conductivity as the transistor 102 and the like, theinvention is not limited to this. Any element having rectification canbe adopted. For example, instead of the transistor 110, a PN junctiondiode, a PIN junction diode, a Schottky diode, a diode-connectedtransistor having the opposite conductivity to that of the transistor102 and the like may be adopted. That is, the potential at the terminal107 has only to be prevented from dropping to a certain level.

However, it is desirable that the transistors 110 and 102 have the sameconductivity and the same threshold voltage in rough. This is because inthe case where the threshold voltage of the transistor 110 is differentfrom that of the transistor 102, the transistor 102 may be turned ONwhen an H signal (high potential side power supply VDD) is input to theinput terminal 105 and an L signal (low potential side power supply GND)is input to the input terminal 106. Thus, characteristics of thetransistors 110 and 102 are desirably set to be uniform by disposingthem adjacently to each other and the like. In the case of crystallizingtheir semiconductor layers by laser irradiation for example, thetransistors 110 and 102 are desirably irradiated with the same shot.However, the threshold voltages of the transistors 110 and 102 may havesome small variations as long as they have no influence on theoperation.

It is to be noted that described in this embodiment is the modifiedexample of the circuit shown in Embodiment Mode 1. Thus, the descriptionof Embodiment Mode 1 can be applied to this embodiment as well.

Embodiment Mode 3

Described in this embodiment mode is the inverter circuit for dealingwith the first and second problems described in the section of theproblems to be solved by the invention, which is obtained by modifyingthe circuit described in Embodiment Mode 1 In this embodiment mode, aninverter circuit for dealing with the first problem is described bymodifying the circuit in FIG. 34.

FIG. 8 shows a modified inverter circuit of FIG. 34. A diode-connectedtransistor 801 is connected in series to a transistor 3409. It is to benoted that although the transistor 801 is disposed between the drainterminal of the transistor 3409 and the terminal 3407, the invention isnot limited to this. For example, it may be connected to the sourceterminal side of the transistor 3409.

As described above, by disposing the transistor 801, the potential atthe terminal 3407 is prevented from dropping to a large degree.Therefore, the potential at the terminal 3407 is increased quickly. As aresult, the transistor 3401 is instantly turned OFF, thus the terminal3407 is brought into the floating state. At this point, the transistor3402 is ON as the potential at the terminal 3408 is still on theincrease. Therefore, it becomes possible to deal with the problem thatthe potentials at the terminals 3408 and 3407 are not increasedsufficiently.

Due to the transistor 801, the potential change at the terminal 3407 canbe suppressed, thus the potential can change instantly. This contributesto the faster circuit operation.

By the configuration as described above, both of the first and secondproblems described in the section of the problems to be solved by theinvention can be solved at the same time.

It is to be noted that although N-channel type transistors are employedin FIG. 8, the invention is not limited to this. P-channel typetransistors may be employed to configure the circuit, or a CMOS typecircuit may be employed as well. When adopting P-channel typetransistors for all of the transistors in the circuit shown in FIG. 8,the potentials of VDD and GND may be replaced with each other.

It is to be noted that although the transistor 801 shown in FIG. 8 hasthe same conductivity as the transistor 3402 and the like, the inventionis not limited to this. Any element having rectification may be adopted.For example, instead of the transistor 801, a PN junction diode, a PINjunction diode, a Schottky diode, a diode-connected transistor havingthe opposite conductivity to that of the transistor 3402 and the likemay be adopted. That is, the potential at the terminal 3407 has only tobe prevented from dropping to a certain level.

However, it is desirable that the transistors 801 and 3402 have the sameconductivity and the same threshold voltage in rough. This is because inthe case where the threshold voltage of the transistor 801 is differentfrom that of the transistor 3402, the transistor 3402 may be turned ONwhen an H signal (high potential side power supply VDD) is input to theinput terminal 3405 and an L signal (low potential side power supplyGND) is input to the input terminal 3406. Thus, characteristics of thetransistors 801 and 3402 are desirably set to be uniform by disposingthem adjacent to each other and the like. In the case of crystallizingtheir semiconductor layers by laser irradiation for example, thetransistors 801 and 3402 are desirably irradiated with the same shot.However, the threshold voltages of the transistors 801 and 3402 may havesome small variations as long as they have no influence on theoperation.

Embodiment Mode 4

Described in Embodiment Modes 1 to 3 is the case of applying theinvention to the inverter circuit. In this embodiment, the case wherethe invention is applied to a circuit other than the inverter circuit isdescribed.

FIG. 10 shows the configuration of a clocked inverter circuit to whichthe invention is applied. The circuit shown in FIG. 10 is configured byextending the inverter circuit shown in FIG. 2. However, it is alsopossible to configure a clocked inverter circuit by extending thealternative circuit shown in any one of Embodiment Modes 1 to 3.

In FIG. 10, transistors 1002B and 1003B control whether or not to outputa signal to an output terminal of the clocked inverter circuit.Generally, ON/OFF is controlled in synchronism with a clock signal, asampling pulse signal and the like. Thus, the transistors 1002B and1003B are simultaneously turned ON/OFF in synchronism with a signalinput to an input terminal 1005B. On the other hand, transistors 1002and 1003 invert an input signal which is input to an input terminal 1005so as to be output to an output terminal 1010.

As shown in FIG. 10, the amplitude of an output signal is prevented frombeing decreased by using transistors 1001, 1009, 1001B and 1009B,capacitors 1004 and 1004B and the like to P-channel transistors in thecase of configuring a CMOS type clocked inverter. Although the gateterminal of the transistor 1003B is connected to the input terminal1005B in FIG. 10, the invention is not limited to this. The gateterminal of the transistor 1003B may be connected to a terminal 1007B.

Alternatively, a diode-connected transistor may be connected in seriesto the transistors 1009, 100913 and the like as in FIG. 1. It is alsopossible to configure a clocked inverter by extending the invertercircuit shown in FIG. 8 by changing the connection of the transistors1001 and 1001B to that of the transistor 3401 shown in FIG. 8.

The operation of the circuit in FIG. 10 is the same as those describedin Embodiment Modes 1 to 3, therefore, it is omitted herein.

Hereupon, a symbol 1101 representing the clocked inverter of thisembodiment mode is shown in FIG. 11. A terminal 1105 corresponds to theterminal 1005B and a terminal 1106 corresponds to the terminal 1006B. Apair of signals inverted from each other is input to each of theterminals 1105 and 1106. When an H signal is input to the terminal 1105,the signal is output to an output terminal 1102. An input terminal 1103corresponds to the terminal 1006 and an input terminal 1104 correspondsto the terminal 1005. Taking account of this circuit as a clockedinverter circuit, a signal input to the input terminal 1103 is invertedand output to the output terminal 1102. Therefore, the terminal 1103corresponds to an input terminal of the clocked inverter circuit. A pairof signals inverted from each other is input to each of the terminals1103 and 1104.

FIG. 12 shows the configuration of a NAND circuit to which the inventionis applied. The circuit shown in FIG. 12 is configured by extending theinverter circuit shown in FIG. 2. However, it is also possible toconfigure a NAND circuit by extending the alternative circuit shown inany one of Embodiment Mode 1 to 3.

In FIG. 12, the amplitude of an output signal is prevented from beingdecreased by using transistors 1201, 1209, 1201B and 1209B, capacitors1204 and 1204B and the like to P-channel type transistors, namely totransistors 1202 and 1202B in the case of configuring a CMOS type NANDcircuit. As for N-channel type transistors in the case of configuring aCMOS NAND circuit, namely transistors 1203 and 1203B are used withoutany modifications.

Alternatively, a diode-connected transistor may be connected in seriesto the transistors 1209 and 1209B and the like. It is also possible toconfigure a NAND circuit by extending the inverter circuit in FIG. 8 bychanging each configuration of the transistors 1201 and 1201B to that ofthe transistor 3401 in FIG. 8.

The operation of the circuit in FIG. 12 is the same as those describedin Embodiment Modes 1 to 3, therefore, it is omitted herein.

Hereupon, a symbol 1301 representing the NAND circuit of this embodimentmode is shown in FIG. 13. An input terminal 1303 corresponds to theterminal 1206 and an input terminal 1305 corresponds to the terminal1206B. An input terminal 1304 corresponds to the terminal 1205 and aninput terminal 1306 corresponds to the terminal 1205B. A pair of signalsinverted from each other is input to each of the terminals 1303 and1304, and a pair of signals inverted from each other is input to each ofthe terminals 1305 and 1306. An output terminal 1302 corresponds to theterminal 1201. Taking account of the logic operation of this circuit asa NAND circuit, each of the terminals 1303 and 1305 corresponds to aninput terminal of a NAND circuit.

FIG. 14 shows the configuration of a NOR circuit to which the inventionis applied. The circuit shown in FIG. 14 is configured by extending theinverter circuit shown in FIG. 2. However, it is also possible toconfigure a NOR circuit by using the alternative circuit shown in anyone of Embodiment Modes 1 to 3.

In FIG. 14 also, the amplitude of an output signal is prevented frombeing decreased by using transistors 1401, 1409, 1401B and 1409B,capacitors 1404 and 1404B and the like to P-channel type transistors,namely to transistors 1402 and 1402B in the case of configuring a CMOStype NOR circuit. As for N-channel type transistors in the case ofconfiguring a CMOS type NOR circuit, namely transistors 1403 and 1403Bare used without any modifications.

Alternatively, a diode-connected transistor may be connected in seriesto the transistors 1409, 1409B and the like. It is also possible toconfigure a NOR circuit by extending the inverter circuit in FIG. 8 bychanging the connection of the transistors 1401 and 1401B to that of thetransistor 3401 in FIG. 8.

The operation of the circuit in FIG. 14 is the same as those describedin Embodiment Modes 1 to 3, therefore, it is omitted herein.

FIG. 15 shows the configuration of a transfer gate circuit (analogswitch circuit) to which the invention is applied. The circuit in FIG.15 is configured by extending the inverter circuit shown in FIG. 2.However, it is also possible to configure a transfer gate circuit byextending the alternative circuit shown in any one of Embodiment Modes 1to 3.

In the case of FIG. 15, which potential at the terminal 1510 or 1511becomes higher is dependent on the condition. Thus, it is not clearwhich terminal corresponds to the source terminal. Thus, in FIG. 15, atransistor 1502 and a transistor 1502B are disposed in parallel to eachother, and a capacitor 1504 and a capacitor 1504B are disposed in adifferent connection. Therefore, the potentials at the gate terminals ofthe transistors 1502 and 1502B can be increased sufficiently regardlessof which potential at the terminal 1510 or 1511 is lower.

Thus, in the case of a CMOS type transfer gate circuit, the amplitude ofan output signal is prevented from being decreased by using transistors1501, 1509, 1501B and 1509B, capacitors 1504 and 1504B and the like toboth P-channel type and N-channel type transistors, not only toP-channel type transistors. In this manner, by connecting adiode-connected transistor, a capacitor and the like to a transistor inwhich the amplitude of an output signal is decreased, a normal circuitoperation is achieved.

Alternatively, a diode-connected transistor may be connected in seriesto the transistors 1509, 1509B and the like as in FIG. 1. It is alsopossible to configure a transfer gate circuit by extending the invertercircuit in FIG. 8 by changing the connection of the transistors 1501 and1501B to that of the transistor 3401 in FIG. 8.

The operation of the circuit in FIG. 15 is the same as those describedin Embodiment Modes 1 to 3, therefore, it is omitted herein.

Although N-channel type transistors are employed in FIGS. 10, 12, 14 and15, the invention is not limited to them. When employing P-channeltransistors for all the transistors in the circuits shown in FIGS. 10,12, 14 and 15, the potentials of VDD and GND may be replaced with eachother.

Various circuits such as a NAND circuit to which the invention isapplied have heretofore been described in this embodiment mode; however,the application of the invention is not limited to them. It can beapplied to other various circuits.

It is to be noted that described in this embodiment mode are theextended circuits of the ones described in Embodiments 1 to 3.Therefore, the description of Embodiment Modes 1 to 3 can all be appliedto this embodiment mode.

Embodiment Mode 5

In Embodiment Mode 1, it is described that not only the terminal 108 butalso the terminal 107 may be employed as an output terminal of theinverter circuit in FIG. 2. According to the present embodiment mode,various circuit configurations are described by utilizing an output ofthe output terminal 107. That is, described here is the case in whichvarious circuits are operated by operating an inverter circuit whichoutputs a signal from the terminal 108 as a level correction circuit.

First, FIG. 16 shows an inverter circuit to which the invention isapplied. In FIG. 16, the inverter circuit shown in FIG. 1 is used as alevel correction circuit and the terminal 107 is used as an outputterminal so as to be connected to an input terminal of another circuit(inverter circuit here). The circuit (inverter circuit here) is operatednormally by using a signal output from a level correction circuit 1601.

Input terminals 1603 and 1604 of the level correction circuit 1601 areconnected to the terminals 105 and 106 respectively. An output terminal1605 of the level correction circuit 1601 is connected to the terminal107 and an output terminal 1606 is connected to the terminal 106.

A pair of signals inverted from each other is input to each of the inputterminals 1603 and 1604. Then, the signal from the input terminal 1604is directly output to the output terminal 1606, while a signal from theinput terminal 1603 is output to the output terminal 1605 after itspotential is adjusted. Specifically, in the case of an H signal, thehigher potential is output.

Thus, in the case of configuring a CMOS type inverter circuit, theamplitude of an output signal is prevented from being decreased byconnecting a P-channel type transistor to the output terminal 1605.

In FIG. 16, the output terminal 1605 of the level correction circuit1601 is connected to the gate terminal of a transistor 1608 and theoutput terminal 1606 is connected to the gate terminal of a transistor1609. As a result, a signal is output to an output terminal 1607 withoutbeing reduced of its amplitude.

As described above, in the case of configuring a CMOS type circuit, asignal from the output terminal 1605 is input to the gate terminal of aP-channel type transistor. As a result, a normal circuit operation isachieved.

The configuration of the level correction circuit is not limited to thatshown in FIG. 16. The circuits described in any one of Embodiment Modes1 to 3 can be used arbitrarily.

When representing the circuit in FIG. 16 by the symbol 301 shown in FIG.3, the terminal 1604 corresponds to the terminal 303, the terminal 1603corresponds to the terminal 304 and the terminal 1607 corresponds to theterminal 302.

Similarly, FIG. 17 shows the configuration of a clocked inverter towhich the invention is applied. Transistors 1702 and 1705 aresimultaneously turned ON/OFF by using a level correction circuit 1601Cand transistors 1703 and 1704 are controlled by using a level correctioncircuit 1601A.

Since the gate terminals of the transistors 1702 and 1703 may besupplied with high potentials, the amplitude of an output signal isprevented from being decreased.

When representing the circuit in FIG. 17 by the symbol 1101 showing theclocked inverter circuit in FIG. 11, a terminal 1604A corresponds to theterminal 1103, a terminal 1603A corresponds to the terminal 1104 and aterminal 1604C corresponds to the terminal 1106. In addition, a terminal1603C corresponds to the terminal 1105 and a terminal 1706 correspondsto the terminal 1102.

Similarly, FIG. 18 shows the configuration of a NAND circuit to whichthe invention is applied. Transistors 1802 and 1805 are controlled byusing a level correction circuit 1601B and transistors 1803 and 1804 arecontrolled by using the level correction circuit 1601A.

Since the gate terminals of the transistors 1802 and 1803 can besupplied with high potentials, the amplitude of an output signal isprevented from being decreased.

When representing the circuit in FIG. 18 by the symbol 1301 showing theNAND circuit in FIG. 13, the terminal 1604A corresponds to the terminal1303 and the terminal 1603A corresponds to the terminal 1304 and aterminal 1604B corresponds to the terminal 1105. In addition, a terminal1603B corresponds to the terminal 1306 and a terminal 1806 correspondsto the terminal 1302.

Similarly, FIG. 19 shows the configuration of a NOR circuit to which theinvention is applied. Transistors 1902 and 1905 are controlled by usingthe level correction circuit 1601B and transistors 1903 and 1904 arecontrolled by using the level correction circuit 1601A.

Since the gate terminals of the transistors 1902 and 1903 can besupplied with high potentials, the amplitude of an output signal of theoutput terminal 1906 is prevented from being decreased.

Similarly, the configuration of a transfer gate circuit to which theinvention is applied is shown in FIG. 20. A transistor 2003 iscontrolled by using the level correction circuit 1601A.

Since the gate terminal of the transistor 2002 can be supplied with ahigh potential, the amplitude of a signal from input/output terminals2003 and 2004 is prevented from being decreased.

Described heretofore is the case of disposing one output terminal asshown in FIGS. 16 to 20. However, when connecting another circuit nextto the circuit, an inverted signal is frequently required. Hereupon,examples of disposing two output terminals and outputting an invertedsignal are described below.

FIG. 21 shows the configuration of an inverter to which the invention isapplied. One inverter circuit includes transistors 2103 and 2104 whileanother inverter circuit includes transistors 2103B and 2104B. When apair of signals inverted from each other is input to each of theinverter circuits, a pair of signals inverted from each other can beoutput.

However, each of the gate terminals of the transistors 2103 and 2103B isrequired to be input with a potential which is higher than VDD. Further,each of the gate terminals of the transistors 2103 and 2103B is requiredto be input with a pair of signals inverted from each other. Thus, thetwo level correction circuits 1601A and 1601B are required.

FIG. 22 shows a symbol 2201 representing the circuit in FIG. 21. Asignal input to an input terminal 2203 is inverted and output to anoutput terminal 2202. An inverted signal of the signal at the inputterminal 2203 is input to an input terminal 2204, and an inverted signalof the signal at the output terminal 2202 is output to an outputterminal 2207. Therefore, the terminal 1604A corresponds to the terminal2203 and the terminal 1603A corresponds to the terminal 2204. Inaddition, the terminal 2106 corresponds to the terminal 2202 and aterminal 2106B corresponds to the terminal 2207.

Similarly, FIG. 23 shows the configuration of a clocked inverter towhich the invention is applied. One clocked inverter circuit includestransistors 2302, 2303, 2304 and 2305 while another clocked invertercircuit includes transistors 2302B, 2303B, 2304B and 2305B. When each ofthe clocked inverter circuits is input with a pair of signals invertedfrom each other, a pair of signals inverted from each other can beoutput.

However, each of the gate terminals of the transistors 2303 and 2303B isrequired to be input with a potential which is higher than VDD. Further,each of the gate terminals of the transistors 2303 and 2303B is requiredto be input with a pair of signals inverted from each other. Thus, thetwo level correction circuits 1601A and 1601B are required.

In addition, each of the gate terminals of the transistors 2302 and2302B is required to be input with a potential which is higher than VDD.However, the gate terminals of the transistors 2302 and 2302B may beinput with the same signal. Thus, the one level correction circuit 1601Cis required.

FIG. 24 shows a symbol 2401 representing the circuit in FIG. 23. When anH signal is input to a terminal 2405, a signal from an input terminal2403 is inverted and output to an output terminal 2402. An invertedsignal of the signal at the input terminal 2403 is input to an inputterminal 2404, an inverted signal of the signal at the input terminal2405 is input to an input terminal 2406 and an inverted signal of thesignal at the output terminal 2402 is output to an output terminal 2407.Therefore, the terminal 1603C corresponds to the terminal 2405, theterminal 1604C corresponds to the terminal 2406 and the terminal 1604Acorresponds to a terminal 2403. In addition, the terminal 1603Acorresponds to the terminal 2404, the terminal 2306 corresponds to theterminal 2402 and a terminal 2306B corresponds to the terminal 2407.

Similarly, FIG. 25 shows the configuration of a NAND circuit to whichthe invention is applied. One NAND circuit includes transistors 2502,2503, 2504 and 2505 while another NAND circuit includes transistors2502B, 2503B, 2504B and 2505B. When each of the NAND circuits is inputwith a pair of signals inverted from each other, a pair of signalsinverted from each other can be output. However, each of the gateterminals of the transistors 2502, 2503, 2502B and 2503B is required tobe input with a potential which is higher than VDD. Further, each of thegate terminals of the transistors 2502 and 2502B or each of the gateterminals of the transistors 2503 and 2503B is required to be input witha pair of signals inverted from each other. Thus, the four levelcorrection circuits 1601A, 1601B, 1601D and 1601E are required.

FIG. 26 shows a symbol 2601 representing the circuit in FIG. 25. Signalsfrom input terminals 2603 and 2605 are output to an output terminal2602. An inverted signal of the signal at the input terminal 2603 isinput to an input terminal 2604, an inverted signal of the signal at theinput terminal 2605 is input to an input terminal 2606 and an invertedsignal of the signal at the output terminal 2602 is input to an outputterminal 2607. The terminal 1604B corresponds to the terminal 2603, theterminal 1604A corresponds to the terminal 2605 and the terminal 1603Bcorresponds to the terminal 2604. In addition, the terminal 1603Acorresponds to the terminal 2606, the terminal 2506 corresponds to theterminal 2602 and a terminal 2506B corresponds to the terminal 2607.

Similarly, the invention can be applied to a NOR circuit.

It is to be noted that although a level correction circuit is employedin this embodiment mode to control the potential level, the invention isnot limited to this. The circuit may be operated by directly inputting asignal having a large amplitude. For example, the terminal 1605C inFIGS. 17 and 23 may be input with a signal having a large amplitudewithout using the level correction circuit 1601C, specifically, such asa signal whose potential at H level is higher than VDD. Similarly, asignal having a large amplitude may be directly input to the terminals1605A, 1606A, 1605B and 1606B in FIGS. 17 and 23 without using the levelcorrection circuits 1601A and 1601B.

It is also to be noted that although a signal is input to a circuit tobe operated after controlling a potential level by using a levelcorrection circuit in this embodiment mode, the invention is not limitedto this. Conversely, it is also possible to operate a circuit first, andthen control its potential level. FIG. 27 shows the configuration of aninverter circuit to which the invention is applied. Two pairs ofinverter circuits are configured by using transistors 2708, 2709, 2710and 2711. This is because an inverted signal is also required in a levelcorrection circuit 2701 in the subsequent stage. Signals are input froman input terminal 2703 and an input terminal 2704 for inputting theinverted signal, and output to an output terminal 2707 after the levelsare controlled in the level correction circuit 2701. The invention canbe applied to alternative circuits as well as an inverter.

As described above, the invention can be applied to various circuitssuch as a clocked inverter circuit and a NAND circuit described in thepresent embodiment mode; however, the invention is not limited to them.It can be applied to various alternative circuits.

The things using the circuits explained in Embodiment Modes 1 to 4 aredescribed in this embodiment mode. Therefore, the description inEmbodiment Modes 1 to 4 can be also applied to this embodiment mode, andwith the circuit configurations, a semiconductor device with accurateoperation can be manufactured at low cost.

Embodiment 1

Described in this embodiment is the configuration and operation of adisplay device, a signal line driver circuit and the like. The circuitconfigurations described in Embodiment Modes 1 to 5 can be applied to apart of a signal line driver circuit or a part of a gate line drivercircuit.

Referring to FIG. 28, a display device includes pixels 2801, a gate linedriver circuit 2802 and a signal line driver circuit 2810. The gate linedriver circuit 2802 sequentially outputs selection signals to the pixels2801 and the signal line driver circuit 2810 sequentially outputs videosignals to the pixels 2801. In the pixels 2801, an image is displayed bycontrolling the state of light according to the video signals. Voltageis frequently employed as a video signal input from the signal linedriver circuit 2810 to the pixels 2801. That is, the states of a displayelement disposed in the pixel and an element for controlling the displayelement are changed according to a video signal (voltage) input from thesignal line driver circuit 2810 in many cases. Occasionally, current isinput to the pixels 2801 as a video signal. As examples of the displayelement disposed in each pixel, liquid crystal (LCD), organic EL, anelement for an FED (field emission display), a DMD (digital mirrordevice) or the like can be employed.

It is to be noted that the number of the gate line driver circuit 2802and the signal line driver circuit 2810 may be more than one.

The configuration of the signal line driver circuit 2810 may be dividedinto a plurality of units. For example, it is roughly divided into ashift register 2803, a first latch circuit (LAT1) 2804, a second latchcircuit (LAT 2) 2805, a digital-to-analog converter circuit 2806 and thelike.

The operation of the signal line driver circuit 2810 is described inbrief now. The shift register 2803 includes a plurality of lines of flipflop circuits (FF), latch circuits and the like, and a clock signal(S-CLK) 2812, a start pulse (SP) 2813 and a clock inverted signal(S-CLKb) 2811 are input thereto. According to the timing of thesesignals, sampling pulses are sequentially output.

A sampling pulse output from the shift register 2803 is input to thefirst latch circuit 2804. The first latch circuit 2804 is input with avideo signal from a video signal line 2808, and according to the timingat which the sampling pulse is input, the video signal is held in thefirst latch circuit 2804 in each line. In the case of disposing thedigital-to-analog converter circuit 2806, the video signals are digitalvalues.

When the video signal storage is completed up to the last line in thefirst latch circuit 2804, a latch pulse (Latch Pulse) is input from alatch control line 2809 during a horizontal fry-back period, and thevideo signals held in the first latch circuit 2804 are transferred tothe second latch circuit 2805 all at once. Then, one row of the videosignals held in the second latch circuit 2805 are input to thedigital-to-analog converter circuit 2806 all at one. Signals output fromthe digital-to-analog converter circuit 2806 are then input to thepixels 2801.

While the video signals held in the second latch circuit 2805 are inputto the pixels 2801 through various circuits, the shift register 2803outputs sampling pulses again. That is, two operations are performed atthe same time. Therefore, a line sequential drive is enabled. In thismanner, such operations are repeated.

It is to be noted that when the first latch circuit 2804 and the secondlatch circuit 2805 are capable of storing an analog value, thedigital-to-analog converter circuit 2806 can be omitted in many cases.Meanwhile, when the data output to the pixels 2801 has a binary value,namely a digital value, the digital-to-analog converter circuit 2806 canbe omitted in many cases. The signal line driver circuit 2810incorporates a level shift circuit, a gamma-correction circuit, avoltage-to-current converter circuit, an amplifier circuit and the likein some cases.

In addition, it is possible to omit the first latch circuit 2804 and thesecond latch circuit 2805, and connect the video signal line 2808 to thepixels 2801 through a transfer gate circuit (analog switch circuit). Inthat case, the transfer gate circuit is controlled by a sampling pulseoutput from the shift register 2803.

As described above, the configuration of the signal line driver circuit2810 is not limited to the one shown in FIG. 28, and variousconfigurations can be employed.

On the other hand, since the gate line driver circuit 2802 just outputsselection signals to the pixels 2801 sequentially in many cases, it mayinclude a shift register, a level shifter circuit, an amplifier circuitand the like having the similar configuration as that of the shiftregister 2803 in the signal line driver circuit 2810. However, theconfiguration of the gate line driver circuit 2802 is not limited tothis and various configurations can be employed.

The circuit configurations shown in Embodiment Modes 1 to 5 can beapplied to various circuit parts such as the shift register of thesignal line driver circuit 2810 and the gate line driver circuit 2802,or the first latch circuit (LAT1) 2804 and the second latch circuit 2805of the signal line driver circuit 2810.

Referring now to FIGS. 29 and 30, a DFF circuit (delay flip flopcircuit) used in the shift register, the first latch circuit (LAT1) 2804and the second latch circuit 2805 is described.

In a DFF circuit 2901 shown in FIG. 29, a signal is input to an inputterminal 2904 and the circuit operation is controlled according to asynchronizing signal input to terminals 2906 and 2907. Then, the signalis output to an output terminal 2902. Each of terminals 2904 and 2905 isinput with a pair of signals inverted from each other, and similarly,each of the terminals 2906 and 2907 is input with a pair of signalsinverted from each other. As for an output, each of terminals 2902 and2903 outputs a pair of signals inverted from each other. Similarly, in aDFF circuit 3001 shown in FIG. 30, signals are transmitted betweenterminals 300 to 3007.

In FIG. 29, a circuit for outputting an inverted signal is disposed. Onthe other hand, in FIG. 30, a circuit for outputting no inverted signalis disposed. Therefore, each circuit part is disposed in parallel toeach other for generating an inverted signal.

FIG. 31 shows a part of a shift register which is configured with DFFcircuits and the like. It includes DFF circuits 2901A to 2901D. Thecircuit shown in FIG. 29 or in FIG. 30 may be used as each DFF circuit.A clock signal (S-CLK) 2812 and a clock inverted signal (S-CLKb) areinput to the parts corresponding to the terminals 2906 and 2907 (or theterminals 3006 and 3007), and, in synchronism with these signals, theshift register is operated.

When configuring the first latch circuit (LAT1) 2804 by using DFFcircuits and the like, a sampling pulse which is output from the shiftregister is input to the parts corresponding to the terminals 2906 and2907 (or the terminals 3006 and 3007). In addition, when configuring thesecond latch circuit (LAT2) 2805 by using DFF circuits and the like, alatch pulse (Latch Pulse) is input to the parts corresponding to theterminals 2906 and 2907 (or the terminals 3006 and 3007) from the latchcontrol line 2809.

In the case of employing the circuit shown in FIG. 17 or FIG. 23 as aclocked inverter circuit in the DFF circuit of the shift register, andsetting the signal amplitude of the clock signal (S-CLK) 2812 and theclock inverted signal (S-CLKb) larger than that of the power supplyvoltage, the level correction circuit 1601C in FIG. 17, FIG. 23 and thelike can be omitted.

Similarly, in the case of employing the circuit shown in FIG. 17 or FIG.23 as a clocked inverter circuit in the DFF circuit of the first latchcircuit (LAT1) 2804 or the second latch circuit (LAT2) 2805, and settingthe signal amplitude of the video signal input from the video signalline 2808 and the latch pulse (Latch Pulse) input from the latch controlline 2809 larger than that of the power supply voltage, some of thelevel correction circuits in FIG. 17, FIG. 23 and the like can beomitted.

It is to be noted that any types of transistor can be used for thetransistor in the invention and it may be formed on any types ofsubstrate. Accordingly, it is possible to form the whole circuit shownin FIG. 28 on a glass substrate, a plastic substrate, a single crystalsubstrate, an SOI substrate or the like. Alternatively, a part of thecircuit shown in FIG. 28 may be formed on a certain substrate andanother part of the circuit shown in FIG. 28 may be formed on anothersubstrate. That is, not all part of the circuit shown in FIG. 28 isnecessarily formed on the same substrate. For example, in FIG. 28, it ispossible that the pixels 2801 and the gate line driver circuit 2802 areformed with TFTs on a glass substrate and the signal line driver circuit2810 (or part of it) is formed on a single crystal substrate, therebyconnecting the IC chip onto the glass substrate by COG (Chip On Glass).In place of COG, TAB (Tape Auto Bonding), a printed substrate and thelike may be used as well.

As described above, a semiconductor device having the circuitconfigurations described in Embodiment Modes 1 to 5 can be applied to adisplay device.

Embodiment 2

Electronic devices, using the semiconductor device of the invention,include a video camera, a digital camera, a goggle type display (headmounted display), a navigation system, a sound reproducing device (a caraudio equipment, an audio component stereo and the like), a laptoppersonal computer, a game machine, a portable information terminal (amobile computer, a cellular phone, a portable game machine, anelectronic book and the like), an image reproducing device including arecording medium (more specifically, an apparatus which can reproduce arecording medium such as a digital versatile disc (DVD) and so forth,and includes a display for displaying the reproduced image) or the like.Specific examples of these electronic devices are shown in FIG. 32.

FIG. 32A shows a light emitting device, which includes a housing 13001,a support base 13002, a display portion 13003, a speaker portion 13004,a video input terminal 13005 and the like. The display device using thesemiconductor device of the invention can be applied to the displayportion 13003. According to the invention, the light emitting device asshown in FIG. 32A is completed. Since a light emitting device emitslight by itself, it requires no back light and thus a thinner displayportion than a liquid crystal display is obtained. Note that, the lightemitting device includes all the information display devices forpersonal computers, television broadcast reception, advertisementdisplays and the like.

FIG. 32B shows a digital still camera, which includes a main body 13101,a display portion 13102, an image receiving portion 13103, operatingkeys 13104, an external connection port 13105, a shutter 13106 and thelike. The display device using the semiconductor device of the inventioncan be applied to the display portion 13102. According to the invention,the digital still camera as shown in FIG. 32B is completed.

FIG. 32C shows a laptop personal computer, which includes a main body13201, a housing 13202, a display portion 13203, a keyboard 13204, anexternal connection port 13205, a pointing mouse 13206 and the like. Thedisplay device using the semiconductor device of the invention can beapplied to the display portion 13203. According to the invention, thelight emitting device a shown in FIG. 32C is completed.

FIG. 32D shows a mobile computer, which includes a main body 13301, adisplay portion 13302, a switch 13303, operating keys 13304, an infraredport 13305 and the like. The display device using the semiconductordevice of the invention can be applied to the display portion 13302.According to the invention, the mobile computer as shown in FIG. 32D iscompleted.

FIG. 32E shows a portable image reproducing device provided with arecording medium (specifically, a DVD playback device), which includes amain body 13401, a housing 13402, a display portion A13403, a displayportion B13404, a recording medium (such as a DVD) read-in portion13405, operating keys 13406, a speaker portion 13407 and the like. Thedisplay portion A13403 mainly displays image data and the displayportion B13404 mainly displays text data. The display device using thesemiconductor device of the invention can be applied to the displayportions A13403 and B13404. Note that the image reproducing deviceprovided with a recording medium includes a game machine for domesticuse and the like. According to the invention, the DVD playback deviceshown in FIG. 32E is completed.

FIG. 32F shows a goggle type display (head mounted display), whichincludes a main body 13501, a display portion 13502, and an arm portion13503. The semiconductor device of the invention can be applied to thedisplay portion 13502. According to the invention, the goggle typedisplay as shown in FIG. 32F is completed.

FIG. 32G shows a video camera, which includes a main body 13601, adisplay portion 13602, a housing 13603, an external connection port13604, a remote control receiving portion 13605, an image receivingportion 13606, a battery 13607, an audio input portion 13608, operatingkeys 13609 and the like. The display device using the semiconductordevice of the invention can be applied to the display portion 13602.According to the invention, the video camera as shown in FIG. 32G iscompleted.

FIG. 32H shows a cellular phone, which includes a main body 13701, ahousing 13702, a display portion 13703, an audio input portion 13704, anaudio output portion 13705, an operating keys 13706, an externalconnection port 13707, an antenna 13708 and the like. The display deviceusing the semiconductor device of the invention can be applied to thedisplay portion 13703. Note that, by displaying white characters on ablack background of the display portion 13703, the consumption currentof the cellular phone can be suppressed. According to the invention, thecellular phone as shown in FIG. 32H is completed.

If the higher luminance of a light emitting material becomes availablein the future, the semiconductor device of the invention will beapplicable to a front type or a rear type projector in which lightincluding output image data is enlarged by lenses or the like.

The above-described electronic devices are more likely to be used fordisplaying data that is transmitted through telecommunication paths suchas Internet or a CATV (cable television), in particular for displayingmoving image data. Since a light emitting material exhibits highresponse speed, a light emitting device is suitably used for a movingimage display.

In addition, since a light emitting device consumes power in its lightemitting portion, it is desirable that data is displayed so that thelight emitting portion occupies as small space as possible. Therefore,in the case of using a light emitting device in a display portion thatmainly displays text data such as a cellular phone and a soundreproducing device, it is desirable to drive the device so that textdata is displayed with light emitting parts on a non-emittingbackground.

As described above, an application range of the invention is so widethat the invention can be applied to electronic devices in variousfields. The electronic devices in this embodiment may include a displaydevice using a semiconductor device having any one of configurationsshown in the foregoing Embodiment Modes 1 to 5.

The invention claimed is:
 1. A semiconductor device comprising: a firstthin film transistor, a second thin film transistor, a third thin filmtransistor, and a fourth thin film transistor, wherein: the first thinfilm transistor, the second thin film transistor, the third thin filmtransistor, and the fourth thin film transistor are N-channel type; oneof a source and a drain of the first thin film transistor iselectrically connected to one of a source and a drain of the second thinfilm transistor; one of a source and a drain of the third thin filmtransistor is electrically connected to one of a source and a drain ofthe fourth thin film transistor; the one of the source and the drain ofthe third thin film transistor is electrically connected to a gate ofthe first thin film transistor; the other of the source and the drain ofthe first thin film transistor is electrically connected to the other ofthe source and the drain of the third thin film transistor; the other ofthe source and the drain of the first thin film transistor iselectrically connected to a gate of the third thin film transistor; theother of the source and the drain of the second thin film transistor isconfigured to be input with a first potential; the other of the sourceand the drain of the fourth thin film transistor is configured to beinput with the first potential; a gate of the second thin filmtransistor is electrically connected to a gate of the fourth thin filmtransistor; the other of the source and the drain of the first thin filmtransistor is configured to be input with a first signal; the gate ofthe second thin film transistor is configured to be input with a secondsignal; and the one of the source and the drain of the first thin filmtransistor is configured to output a third signal.
 2. An electronicdevice comprising: the semiconductor device according to claim 1; and anoperating key, an antenna, a keyboard, a battery, an audio inputportion, or a speaker.
 3. A semiconductor device comprising: a firstthin film transistor, a second thin film transistor, a third thin filmtransistor, and a fourth thin film transistor, wherein: the first thinfilm transistor, the second thin film transistor, the third thin filmtransistor, and the fourth thin film transistor are N-channel type; oneof a source and a drain of the first thin film transistor iselectrically connected to one of a source and a drain of the second thinfilm transistor; one of a source and a drain of the third thin filmtransistor is electrically connected to one of a source and a drain ofthe fourth thin film transistor; the one of the source and the drain ofthe third thin film transistor is electrically connected to a gate ofthe first thin film transistor; the other of the source and the drain ofthe first thin film transistor is electrically connected to the other ofthe source and the drain of the third thin film transistor; the other ofthe source and the drain of the first thin film transistor iselectrically connected to a gate of the third thin film transistor; theother of the source and the drain of the second thin film transistor iselectrically connected to a first wiring; the other of the source andthe drain of the fourth thin film transistor is electrically connectedto the first wiring; a gate of the second thin film transistor iselectrically connected to a gate of the fourth thin film transistor; theother of the source and the drain of the first thin film transistor iselectrically connected to a first input terminal; the gate of the secondthin film transistor is electrically connected to a second inputterminal; and the one of the source and the drain of the first thin filmtransistor is electrically connected to an output terminal.
 4. Anelectronic device comprising: the semiconductor device according toclaim 3; and an operating key, an antenna, a keyboard, a battery, anaudio input portion, or a speaker.
 5. A semiconductor device comprising:a first thin film transistor, a second thin film transistor, a thirdthin film transistor, and a fourth thin film transistor, wherein: thefirst thin film transistor, the second thin film transistor, the thirdthin film transistor, and the fourth thin film transistor are N-channeltype; one of a source and a drain of the first thin film transistor iselectrically connected to one of a source and a drain of the second thinfilm transistor; one of a source and a drain of the third thin filmtransistor is electrically connected to one of a source and a drain ofthe fourth thin film transistor; the one of the source and the drain ofthe third thin film transistor is electrically connected to a gate ofthe first thin film transistor; the other of the source and the drain ofthe first thin film transistor is electrically connected to the other ofthe source and the drain of the third thin film transistor; the other ofthe source and the drain of the first thin film transistor iselectrically connected to a gate of the third thin film transistor; theother of the source and the drain of the second thin film transistor iselectrically connected to a first wiring; the other of the source andthe drain of the fourth thin film transistor is electrically connectedto the first wiring; a gate of the second thin film transistor iselectrically connected to a gate of the fourth thin film transistor; theother of the source and the drain of the first thin film transistor isconfigured to be input with a first signal; the gate of the second thinfilm transistor is configured to be input with a second signal; and theone of the source and the drain of the first thin film transistor isconfigured to output a third signal.
 6. An electronic device comprising:the semiconductor device according to claim 5; and an operating key, anantenna, a keyboard, a battery, an audio input portion, or a speaker. 7.A semiconductor device comprising: a first thin film transistor, asecond thin film transistor, a third thin film transistor, and a fourththin film transistor, wherein: the first thin film transistor, thesecond thin film transistor, the third thin film transistor, and thefourth thin film transistor are N-channel type; one of a source and adrain of the first thin film transistor is electrically connected to oneof a source and a drain of the second thin film transistor; one of asource and a drain of the third thin film transistor is electricallyconnected to one of a source and a drain of the fourth thin filmtransistor; the one of the source and the drain of the third thin filmtransistor is electrically connected to a gate of the first thin filmtransistor; the other of the source and the drain of the first thin filmtransistor is electrically connected to the other of the source and thedrain of the third thin film transistor; the other of the source and thedrain of the first thin film transistor is electrically connected to agate of the third thin film transistor; the other of the source and thedrain of the second thin film transistor is configured to be input witha first potential; the other of the source and the drain of the fourththin film transistor is configured to be input with the first potential;a gate of the second thin film transistor is electrically connected to agate of the fourth thin film transistor; the other of the source and thedrain of the first thin film transistor is electrically connected to afirst input terminal; the gate of the second thin film transistor iselectrically connected to a second input terminal; and the one of thesource and the drain of the first thin film transistor is electricallyconnected to an output terminal.
 8. An electronic device comprising: thesemiconductor device according to claim 7; and an operating key, anantenna, a keyboard, a battery, an audio input portion, or a speaker.